The present invention relates to a radiation image recording system mainly used for medical diagnosis and, more particularly, to an image processing apparatus for obtaining image data from an original radiation image formed by transmitting radiation through an object to be examined, and forming a visible image on a final recording medium.
An example of a radiation image recording system using X-rays as radiation will be described with reference to FIG. 1.
The X-ray image recording system comprises an X-ray sensing apparatus 1, an image scanning apparatus 2, an image processing apparatus 3, an image reproducing apparatus 4 and an image recording apparatus 5.
In the X-ray sensing apparatus 1, an object to be examined is irradiated with X-rays, and energy corresponding to the X-rays transmitted through the object is accumulated on a phosphor plate comprising an accumulation type phosphor. Energy distribution on the phosphor plate corresponds to X-ray image data obtained from the X-rays transmitted through the object. When the phosphor plate is irradiated with light length, the accumulated energy is thereby stimulated and light is emitted from the phosphor plate.
The image scanning apparatus 2 radiates an energy-stimulating light beam having a wavelength within the range of between 500 to 800 nm on the phosphor plate in which the energy for forming an X-ray image by the X-ray sensing apparatus 1 is accumulated. Thus, the energy accumulated on the phosphor plate is stimulated, thereby emitting phosphorescent light having a wavelength within the range of between 300 to 500 nm. This light is then detected by a photodetector (e.g., a photoelectron multiplier, a photodiode, or the like) which detects light in the above wavelength range, and the image accumulated on the phosphor plate is obtained.
In the image processing apparatus 3, X-ray image data read by the imagescanning apparatus 2, i.e., an output signal from the photodetector, is nonlinearly amplified and is converted into a digital signal by an analog-to-digital (A/D) converter. Thereafter the digital signal is subjected to frequency emphasis processing and gradation processing, as needed. The resultant data is stored in a storage means such as an image memory in the image processing apparatus 3.
The image reproducing apparatus 4 sequentially reads out digital image data stored in the image memory of the image processing apparatus 3, and converts the readout data into an analog signal with a digital-to-analog (D/A) converter. The analog signal is then amplified by an amplifier and supplied to a recording light emitting element so as to convert the analog image data into optical data.
The image recording apparatus 5 radiates the optical data through, e.g., a lens system, onto a recording medium such as film, forming an X-ray image thereon. The X-ray image formed on the recording medium by the image recording apparatus 5 may be subjected to observation for such use as diagnosis.
The conventional image processing apparatus 3 in the above-mentioned system has a configuration as shown in FIG. 2.
The image processing apparatus 3 comrpises a buffer memory 6 and image processor 7.
The buffer memory 6 is connected to an image data/control bus, and stores X-ray image data. The image processor 7 is connected to a central processing unit (CPA) bus, and carried out frequency emphasis processing and gradation processing, with respect to the X-ray image data stored in the buffer memory 6, in accordance with a command supplied from a CPU (not shown) through the CPU bus. Frequency emphasis processing emphasizes a predetermined special frequency component in the image data, and gradation processing gives predetermined gradation characteristics to the image data.
An algorithm performed by the image processing image processing of the apparatus 3 will be described next.
The image processing is executed by an operation in accordance with the following formula: EQU S=.gamma.[S0+.beta.(S0-Sus)] (1)
In the formula (1).
S0: Original image data read out from the phosphor plate by the image scanning apparatus 2 PA1 Sus: Unsharp mask data represented by EQU Sus=.SIGMA..sub.i,j Si,j/N.sup.2 PA1 .beta.: an emphasis coefficient of a high frequency component PA1 .gamma.: a gradation coefficient
(where Si,j: data for coordinates (i,j) of an original image and N: parameter indicating a mask size), unsharp mask data meaning data of an unsharp image (to be referred to as an "unsharp mask" hereinafter) at every scanning point (i.e., pixel) so that an original image is blurred to include frequency components below a predetermined super-low frequency component
In the formula (1), the operation of [S0+.beta.(S0-Sus)] is known as frequency emphasis processing for emphasizing a predetermined special frequency component. The operation of .gamma.[S0+.beta.(S0-Sus)] is known as gradation processing for changing the gradation of the X-ray image data subjected to frequency emphasis processing in accordance with, e.g., predetermined characteristics (see U.S. Pat. No. 4,387,428). Normally, gradation processing is performed by looking up a given table prepared in advance.
X-ray image data processing based upon the above-mentioned algorithm will be described with reference to FIGS. 3 and 4. FIG. 3 illustrates each pixel data p of X-ray image data when the parameter (mask size) N of the unsharp mask is 9. FIG. 4 schematically shows a microprogram stored in advance in the image processor 7 shown in FIG. 2. Referring to FIG. 3, i and j indicate coordinates of each scanning point, i.e., a pixel.
When processing starts, an intialization start command is supplied from the CPU through the CPU bus to the image processor 7. The processor 7 sets operational parameters, namely, the emphasis coefficient .beta., the gradation coefficient .gamma. and the mask size N in accordance with the processing information set by and supplied from a control console (not shown).
When this setting operation ends, the processor 7 awaits a processing start command from the CPU at an identical program count position on the microprogram. While waiting, the processor 7 is in a hold state.
The CPU checks the standby state of peripheral equipment of the system, and when checking is completed, supplies the processing start command to the processor 7. Upon reception of this command, the processor 7 reads out the X-ray image data stored in the buffer memory 6, and starts the operation based upon the formula (1).
The operation is conducted for each pixel p of the unsharp mask in accordance with the processing routine of the microprogram, and the resultant pixel data is temporarily stored in the memory 6.
Prior to the processing operation, the number of pixels p for one line is set by a store command in the microprogram. The processor 7 repeats the operation and output of pixel data for the number of times corresponding to the preset number of pixels p. Repetitive processing is executed in such a manner that after a certain pixel is processed and the resultant data is output, the microprogram is returned to the start of the operation by a return command, and the process begins again for the next pixel. When the process has been repeated the preset number of times, i.e., one line is completed, the microprogram is returned to its original flow by a load command.
In this manner, when processing for one line is completed, the control is switched to the pixel processing for the next line. The processing, operation and output of pixel data is repeated until the last pixel data p, i.e., S(i,j) is processed.
Note that the time required for switching lines is so short when compared to the time required for the operation and output of pixel data that it can be ignored.
In this manner, the operation and output of pixel data is performed by the image processing apparatus 3. The image data subjected to frequency emphasis processing and gradation processing is converted into optical data by the image reproducing apparatus 4. Thereafter, the optical data in the form of a processed image is recorded on a recording medium by image recording apparatus 5.
In the conventional image processing apparatus 3, in order to allow comparison of processed images, two different sets of values for the emphasis coefficient .beta. and the gradation coefficient .gamma. can be used in the formula (1). In the system comprising an image processing apparatus of this type, two image data subjected to frequency emphasis processing and gradation processing using two emphasis coefficients .beta.1 and .beta.2 and two gradation coefficients .gamma.1 and .gamma.2 in the formula (1) are converted into optical, data by the image reproducing the apparatus 4, and are subjected to recording by the image recording apparatus 5. In the image recording apparatus 5, as shown in FIG. 5, processed images A1 and B1 having different emphasis and gradation coefficients .beta. and .gamma. are formed.
In the image processing apparatus 3 when processing using an unsharp mask having a mask size of N.times.N is conducted with respect to pixel data p having coordinates S(i-((N-1)/2),j-((N-1)/2) as shown in FIG. 3, the unsharp mask data Sus can be calculated only after the processing of the pixel data p having coordinates S(i,j) is completed by the processor 7. The calculated result of the unsharp mask data Sus is subjected to frequency emphasis processing and gradation processing using two emphasis coefficients .beta.1 and .beta.2 and two gradation coefficients .gamma.1 and .gamma.2. As a result, two processed images A and B are formed on a singe recording medium X, as shown in FIG. 5.
In this case, however, since the parameter of the mask size of the unsharp mask of both the processed images A and B formed on the single recording medium X is N, the processed images A and B have no difference in contrast.
If two images having different contrasts are to be obtained by a conventional apparatus, two buffer memories and two image processors must be provided in the image processing apparatus. For this reason, the configuration of the image processing apparatus becomes complex, and processing efficiency and throughput is impaired.